Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
The heads are typically provided with aerodynamically shaped slider assemblies which cause the heads to fly over the surfaces of the discs as a result of air currents established by the rotation of the discs. To limit wear and damage to the heads and the discs, contact between the heads and discs is generally minimized except at such time that the discs are not rotated at a speed sufficient to support the heads, during which time the heads are generally moved to and secured over landing zones of the discs, which are typically located near the innermost radii of the discs.
As part of quality control and ongoing research and development efforts, manufacturers of disc drives continually engage in test programs in which, among other things, the wear of the heads and of the discs in the landing zones is monitored as a function of the number of start ups of a disc drive. One manner of monitoring this wear is to measure the maximum static friction force, generally referred to as "stiction", that is exerted on the discs by the heads just prior to the onset of slippage of the discs along the heads and to measure the kinetic friction force, generally referred to as, simply, "friction", that exists after slippage occurs. These forces generally increase as the heads and disc surfaces are eroded so that the magnitudes of these forces provide an indication of the extent to which wear has occurred.
A problem with using stiction and friction as indicators of disc and head wear is that these effects have been difficult to measure accurately without the removal of a disc drive top cover, which is bolted to a corresponding disc drive base deck, or case. As will be recognized, the case and the top cover cooperate to provide an internally sealed environment for the disc drive necessary to minimize the introduction of contaminants which can adversely affect the performance of the drive.
While removal of the top cover has generally permitted accurate stiction and friction measurements, such methodology has also had a number of associated drawbacks. First, removal of the top cover requires a clean environment if the drive is to be reassembled and reoperated. Further, the disassembly and reassembly steps required to facilitate direct stiction and friction measurements with the top cover removed will invariably lead to damage of at least some of the drives subjected to such methodology. More significantly, however, disassembly and reassembly of a disc drive will typically lead to changes in the mechanical interrelationship between the heads and the discs sufficient to introduce inconsistency in subsequent measurements. This lack of consistency in stiction and friction measurements therefore inherently limits the uses that can be made of the measurements.
While problems that are occasioned by disassembly can be overcome by using the spindle motor upon which the discs are mounted to rotate the discs and measuring motor current and back emf (electromotive force) to determine stiction and friction, such measurements tend to have relatively large experimental errors. For example, such an approach will generally yield stiction measurements having tolerances of the order of +20%; friction measurements are even less precise.
Consequently, while stiction and friction measurements have provided disc drive manufacturers with a useful diagnostic tool, the value of this tool has been limited by practical difficulties that are inherent in making the measurements. Accordingly, there is a need for an improved approach to measuring stiction and friction in a disc drive that overcomes such limitations in the prior art.